Abstract: To effectively separate and recover Co(II) from the leachate of spent lithium-ion battery cathodes, we investigated solvent extraction with quaternary ammonium salt N263 in the sodium nitrite system. N\rmO_2^- combines with Co(II) to form an anion Co(NO₂)₃⁻, and it is then extracted by N263. The extraction of Co(II) is related to the concentration of N\rmO_2^- . The extraction efficiency of Co(II) reaches the maximum of 99.16%, while the extraction efficiencies of Ni(II), Mn(II), and Li(I) are 9.27%‒9.80% under the following conditions: 30vol% of N263 and 15vol% of iso-propyl alcohol in sulfonated kerosene, the volume ratio of the aqueous-to-organic phase is 2:1, the extraction time is 30 min, and 1 M sodium nitrite in 0.1 M HNO₃. The theoretical stages require for the Co(II) extraction are performed in the McCabe–Thiele diagram, and the extraction efficiency of Co(II) reaches more than 99.00% after three-stage counter-current extraction with Co(II) concentration of 2544 mg/L. When the HCl concentration is 1.5 M, the volume ratio of the aqueous-to-organic phase is 1:1, the back-extraction efficiency of Co(II) achieves 91.41%. After five extraction and back-extraction cycles, the Co(II) extraction efficiency can still reach 93.89%. The Co(II) extraction efficiency in the actual leaching solution reaches 100%.